Electrochemical devices employing thin layer electrodes such as lithium electrodes are the subject of intense investigation. Typical electrochemical devices or batteries include a metal (lithium) anode, a transition metal oxide composite cathode, and an electrolyte which is typically a solid or liquid and which includes a dissolved metal salt. These new batteries all rely on the technology based on thin films where energy is stored in compact devices. This has lead to the need to produce thinner and thinner electrode and electrolyte layers. The utilization and handling of such layers becomes more difficult as they become thinner. Typical thin film electrodes and electrolytes are easily damaged during manufacturing when such film layers are assembled to form a cell. Multiple cells are assembled to form batteries. During manufacture and assembly, some layers may become punctured permitting direct contact through the electrolyte layer between the anode and cathode. This causes a short circuit rendering the cell defective. Any battery containing a defective cell is itself defective. Under common usage, the term "battery" is often applied to a single cell having an anode, a cathode and an electrolyte layer, as well as to multiple cells.
It is necessary to test the open circuit voltage (OCV) of each cell immediately after it is made in order to determine if it is defective. This is only practical when a small number of cells are made, as for laboratory evaluation. The OCV for a given cell is compared to that of other cells, usually within the same lot, and also compared to the historical norm for previously made cells. Cells that exhibit an uncharacteristically low OCV are thought to have a short circuit, meaning that there is contact between the cathode and anode across the electrolyte layer. If the OCV continues to drop over time, a short circuit becomes the most probable reason. Such OCV testing is not practical for volume production of batteries. However, in order to have an effectively performing battery, it is necessary that such battery not contain any defective cells, that is, cells having short circuits. In present high volume manufacturing, large numbers of cells are produced then immediately assembled into a battery. Batteries having defective cells are later discovered and disposed of, leading to unnecessary waste.
What is needed is a new method to detect defective cells which is applicable to high volume production; which is a non-destructive method to detect faulty or defective cells: and which overcomes problems of including such defective cells in a battery.